1. Field of the Invention
The present invention relates generally to semiconductor technology for programming non-volatile memory devices.
2. Description of the Related Art
Semiconductor memory devices have become more popular for use in various electronic devices. For example, non-volatile semiconductor memory is used in cellular telephones, digital cameras, personal digital assistants, mobile computing devices, non-mobile computing devices and other devices. Electrical Erasable Programmable Read Only Memory (EEPROM), including flash EEPROM, and Electronically Programmable Read Only Memory (EPROM) are among the most popular non-volatile semiconductor memories.
One example of a flash memory system uses the NAND structure, which includes arranging multiple transistors in series, sandwiched between two select gates. The transistors in series and the select gates are referred to as a NAND string. FIG. 1 is a top view showing one NAND string. FIG. 2 is an equivalent circuit thereof. The NAND string depicted in FIGS. 1 and 2 includes four transistors 100, 102, 104 and 106 in series and sandwiched between a first select gate 120 and a second select gate 122. Select gate 120 connects the NAND string to bit line 126. Select gate 122 connects the NAND string to source line 128. Select gate 120 is controlled by applying appropriate voltages to control gate 120CG via selection line SGD. Select gate 122 is controlled by applying the appropriate voltages to control gate 122CG via selection line SGS. Each of the transistors 100, 102, 104 and 106 includes a control gate and a floating gate, forming the gate elements of a memory cell. For example, transistor 100 has control gate 100CG and floating gate 100FG. Transistor 102 includes control gate 102CG and a floating gate 102FG. Transistor 104 includes control gate 104CG and floating gate 104FG. Transistor 106 includes a control gate 106CG and a floating gate 106FG. Control gate 100CG is connected to word line WL3, control gate 102CG is connected to word line WL2, control gate 104CG is connected to word line WL1, and control gate 106CG is connected to word line WL0.
FIG. 3 provides a cross-sectional view of the NAND string described above. As depicted in FIG. 3, the transistors of the NAND string are formed in p-well region 140. Each transistor includes a stacked gate structure that consists of the control gate (100CG, 102CG, 104CG and 106CG) and a floating gate (100FG, 102FG, 104FG and 106FG). The floating gates are formed on the surface of the p-well on top of an oxide film. The control gate is above the floating gate, with an inter-polysilicon dielectric layer separating the control gate and floating gate. The control gates of the memory cells (100, 102, 104, 106) form the word lines. N+ diffused layers 130, 132, 134, 136 and 138 are shared between neighboring cells, whereby the cells are connected to one another in series to form a NAND string. These N+ diffused layers form the source and drain of each of the cells. For example, N+ diffused layer 130 serves as the drain of transistor 122 and the source for transistor of 106, N+ diffused layer 132 serves as the drain for transistor 106 and the source for transistor 104, N+ diffused region 134 serves as the drain for transistor 104 and the source for transistor 102, N+ diffused region 136 serves as the drain for transistor 102 and the source for transistor 100, and N+ diffused layer 138 serves as the drain for transistor 100 and the source for transistor 120. N+ diffused layer 126 connects to the bit line for the NAND string, while N+ diffused layer 128 connects to a common source line for multiple NAND strings.
Note that although FIGS. 1–3 show four memory cells in the NAND string, the use of four transistors is only provided as an example. A NAND string can have less than four memory cells or more than four memory cells. For example, some NAND strings will include 8 memory cells, 16 memory cells, 32 memory cells, etc. The discussion herein is not limited to any particular number of memory cells in a NAND string. A typical architecture for a flash memory system using a NAND structure will include several NAND strings. For example, FIG. 4 shows three NAND strings 202, 204 and 206 of a memory array having many more NAND strings. Each of the NAND strings of FIG. 3 includes two select transistors or gates and four memory cells. For example, NAND string 202 includes select transistors 220 and 230, and memory cells 222, 224, 226 and 228. NAND string 204 includes select transistors 240 and 250, and memory cells 242, 244, 246 and 248. Each string is connected to the source line by one select gate (e.g. select gate 230 and select gate 250). A selection line SGS is used to control the source side select gates. The various NAND strings are connected to respective bit lines by select gates 220, 240, etc., which are controlled by select line SGD. In other embodiments, the select lines do not necessarily need to be in common. Word line WL3 is connected to the control gates for memory cell 222 and memory cell 242. Word line WL2 is connected to the control gates for memory cell 224 and memory cell 244. Word line WL1 is connected to the control gates for memory cell 226 and memory cell 246. Word line WL0 is connected to the control gates for memory cell 228 and memory cell 248. As can be seen, a bit line and respective NAND string comprise a column of the array of memory cells. The word lines (WL3, WL2, WL1 and WL0) comprise the rows of the array. Each word line connects the control gates of each memory cell in the row. For example, word line WL2 is connected to the control gates for memory cells 224, 244 and 252.
Each memory cell can store data (analog or digital). When storing one bit of digital data, the range of possible threshold voltages of the memory cell is divided into two ranges which are assigned logical data “1” and “0.” In one example of a NAND type flash memory, the threshold voltage is negative after the memory cell is erased, and defined as logic “1.” The threshold voltage after a program operation is positive and defined as logic “0.” When the threshold voltage is negative and a read is attempted by applying 0 volts to the control gate, the memory cell will turn on to indicate logic one is being stored. When the threshold voltage is positive and a read operation is attempted by applying 0 volts to the control gate, the memory cell will not turn on, which indicates that logic zero is stored. A memory cell can also store multiple levels of information, for example, multiple bits of digital data. In the case of storing multiple levels of data, the range of possible threshold voltages is divided into the number of levels of data. For example, if four levels of information are stored, there will be four threshold voltage ranges assigned to the data values “11”, “10”, “01”, and “00.” In one example of a NAND type memory, the threshold voltage after an erase operation is negative and defined as “11.” Three different positive threshold voltages are used for the states of “10”, “01”, and “00.”
Relevant examples of NAND type flash memories and their operation are provided in the following U.S. patents/patent applications, all of which are incorporated herein by reference: U.S. Pat. No. 5,570,315; U.S. Pat. No. 5,774,397, U.S. Pat. No. 6,046,935, U.S. Pat. No. 6,456,528 and U.S. patent application. Ser. No. 09/893,277 (Publication No. US2003/0002348).
When programming a flash memory cell, a program voltage is applied to the control gate (via a selected word line) and the bit line is grounded. Electrons from the p-well are injected into the floating gate. When electrons accumulate in the floating gate, the floating gate becomes negatively charged and the threshold voltage of the cell is raised. When the threshold voltage of a memory cell reaches a target verify voltage for its intended state, the cell is locked-out from further programming since the appropriate amount of negative charge has been placed at its floating gate. The floating gate charge and threshold voltage of the cell can be indicative of a particular state corresponding to stored data.
Another type of memory cell utilizes a non-conductive dielectric material in place of a conductive floating gate to store charge in a non-volatile manner. Such a cell is described in an article by Chan et al., “A True Single-Transistor Oxide-Nitride-Oxide EEPROM Device,” IEEE Electron Device Letters, Vol. EDL-8, No. 3, March 1987, pp. 93–95. A triple layer dielectric formed of silicon oxide, silicon nitride and silicon oxide (“ONO”) is sandwiched between a conductive control gate and a surface of a semi-conductive substrate above the memory cell channel. The cell is programmed by injecting electrons from the cell channel into the nitride, where they are trapped and stored in a limited region. This stored charge then changes the threshold voltage of a portion of the channel of the cell in a manner that is detectable. The cell is erased by injecting hot holes into the nitride. See also Nozaki et al., “A 1-Mb EEPROM with MONOS Memory Cell for Semiconductor Disk Application,” IEEE Journal of Solid-State Circuits, Vol. 26, No. 4, April 1991, pp. 497–501, which describes a similar cell in a split-gate configuration where a doped polysilicon gate extends over a portion of the memory cell channel to form a separate select transistor. The foregoing two articles are incorporated herein by reference in their entirety. The programming techniques mentioned in section 1.2 of “Nonvolatile Semiconductor Memory Technology,” edited by William D. Brown and Joe E. Brewer, IEEE Press, 1998, incorporated herein by reference, are also described in that section to be applicable to dielectric charge-trapping devices.
Another approach to storing two bits in each cell has been described by Eitan et al., “NROM: A Novel Localized Trapping, 2-Bit Nonvolatile Memory Cell,” IEEE Electron Device Letters, vol. 21, no. 11, November 2000, pp. 543–545. An ONO dielectric layer extends across the channel between source and drain diffusions. The charge for one data bit is localized in the dielectric layer adjacent to the drain, and the charge for the other data bit localized in the dielectric layer adjacent to the source. Multi-state data storage is obtained by separately reading binary states of the spatially separated charge storage regions within the dielectric.
As device dimensions and the margins between adjacent threshold distributions continue to decrease to provide ever expanding memory capacity, errors and lack of precision in programming and reading back programmed data can become more problematic. For example, a slight shift in charge for a memory cell after completing programming can lead to erroneous readings of the data stored therein when the margins between states is minimal. As devices continue to be scaled, the occurrence of shifts in charge after programming may increase. Unintended changes to the charge stored in the floating gate of a particular memory cell can lead to erroneous readings or the need for error correction codes (ECC) in the memory system.